1. Field of the Invention
The present invention relates to a sheet feeding apparatus which feeds sheets and an image forming apparatus such as a copier, a facsimile, a printer or a multi function peripheral (MFP) having functions of these apparatuses which is equipped in a main body with the sheet feeding apparatus.
2. Related Background Art
A sheet feeding apparatus is conventionally assembled, for example, in a main body of an image forming apparatus and feeds sheets to image forming means which forms an image. The image forming apparatus forms images on individual sheets. A sheet separating mechanism which separates the sheets one by one is disposed in the sheet feeding apparatus assembled in such an apparatus which carries out any treatment on the individual sheets.
As an example of this sheet separating mechanism, there is a slant surface type sheet separating mechanism which separates sheet by utilizing a slant surface. This slant surface type sheet separating mechanism is configured to stop a leading end of an uppermost sheet fed (sent) by a sheet feeding roller with the slant surface and guide the uppermost sheet along the slant surface. Should a plurality of sheets be sent out following the uppermost sheet sent by the sheet feeding roller, the sheet separating mechanism stops and disposes leading ends of the plurality of sheets with the slant surface, thereby feeding the sheets one by one in a separated condition.
Usable as sheets which can be separated with the sheet separating mechanism are various kinds of articles such as paper, resin sheets, thin paper, thick paper, postal cards, letters and the like.
Referring now to FIG. 9, description will be made of a configuration of a sheet feeding apparatus in which the slant surface type sheet separating mechanism is assembled.
A sheet feeding apparatus 101 is composed of a sheet feeding tray unit 102 in which sheets to be fed are mounted, a sheet feeding unit 103 which is to feed the mounted sheets consecutively, a conveying unit 104 which conveys the fed sheets to an image forming unit 3, for example, of an image forming apparatus 1 shown in FIG. 1 and the like.
The sheet feeding tray unit 102 is composed of a sheet feeding tray 121 on which sheets S are to be mounted, a separating slant surface plate 122 which stops, disposes and separates the sheets at a sheet feeding time while guiding the sheets obliquely, and the like. The separating slant surface plate 122 is the slant surface type sheet separating mechanism.
The sheet feeding unit 103 is composed of a sheet feeding arm 114 which is rotatably attached to a driving shaft 113, a driving gear 112 which is integrated with the driving shaft 113, a sheet feeding roller 105 rotatably disposed at a rocking end of the sheet feeding arm 114, a following gear 123 which is integrated with the sheet feeding roller 105, a gear train 124 which consists of six idler gears 111, 110, 109, 108, 107 and 106 which are provided on the sheet feeding arm 114 to transfer the rotation of the driving gear 112 to the following gear 123, and the like.
The conveying unit 104 is composed of a lower guide 115 which guides a bottom surface of a fed sheet, an upper guide 116 which is integrated with the lower guide 115 and guides a top surface of the fed sheet, a roller holder 119 which is rotatably supported by the upper guide 116, a conveying roller 117 which is disposed on the lower guide 115 and conveys the fed sheet in a predetermined direction, a conveying roller 118 which is rotatably disposed in the roller holder 119 in opposition to the conveying roller 117, a conveying spring 120 which urges the roller holder 119 so that the conveying roller 118 is pressed to the conveying roller 117 and the like.
Now, feeding operations of the sheet feeding apparatus will be described on the basis of FIGS. 9 to 11.
At a sheet feeding time and a stop time, the driving shaft 113 is rotated and stopped under rotating control with a controllable driving mechanism (not shown). When sheet feeding is to be started, the driving shaft 113 is rotated with a rotating drive force of the driving mechanism (not shown). This rotation is transmitted from the driving gear 112 to the following gear 123 by way of the gear train 124, whereby the sheet feeding roller 105 starts rotating. By urging means (not shown) and due to weights of the sheet feeding arm 114, the sheet feeding roller 105, the gear train 124 and the like, the sheet feeding arm 114 which is holding the sheet feeding roller 105 revolvably around the driving shaft 131 is subjected to a force in a direction to be rotated counterclockwise in the drawings. Due to this force, the sheet feeding roller 105 is in light pressure contact with a top surface of a sheet which is uppermost out of sheets mounted on the sheet feeding tray 121.
Accordingly, exerted to the sheet is a feeding force which is produced in a sheet feeding direction by a frictional force when the sheet feeding roller 105 starts rotating. At this time, the sheet is pressed to the separating slant surface plate 122 and subjected to a reaction force F. That is, a leading end portion of the sheet is subjected to the feeding force of the sheet feeding roller 105 while being pressed to the separating slant surface plate 122. As a result, the leading end portion of the sheet is curved. In a condition where the sheet is curved, the leading end of the sheet slides along the separating slant surface plate 122 toward the lower guide 115. When the sheets are to be sent out in an overlapped condition at this time, leading ends of the overlapped sheets are separated by the separating slant surface plate 122 and only one sheet is sent out. An advancing direction of the sheet is determined by the upper guide 116 and the lower guide 115, whereby the sheet enters a nip formed between the conveying roller 118 urged toward the carrying roller 117 by the carrying spring 120 and the conveying roller 117, and further conveyed by the conveying roller 117 in a direction indicated by an arrow A.
When a thick sheet having a high rigidity is to be fed, however, the conventional sheet feeding apparatus 101 may hardly curve the leading end portion of the thick sheet having the high rigidity with the separating slant surface plate 122, thereby allowing a sheet feeding pressure to be enhanced autonomously by rotations of a motor of the driving mechanism which rotates the driving shaft 113 and increasing a torque which is required for driving the sheet feeding roller 105.
Accordingly, the conventional sheet feeding apparatus uses a motor which has a large output torque so that even the thick sheet having the high rigidity can be fed by rotating the sheet feeding roller 105. However, the motor which has the large output torque is expensive and large. Accordingly, the conventional sheet feeding apparatus is expensive and may be large due to a large space required for installing the motor.
Furthermore, a small angle xcex1 is formed between the sheet feeding arm 114 and the sheet feeding tray 121 as shown in FIG. 10 when a large number of sheets are mounted on the sheet feeding trays 121. The sheet feeding roller 105 is subjected to the reaction force F of the sheet feeding force when rotated. This reaction force F is divided into a component of force N in a direction of the sheet feeding pressure and a component of force T in a direction along a longitudinal direction of the sheet feeding arm 114. However, the force N in the direction of the sheet feeding pressure is low since the angle xcex1 is small. Accordingly, a sheet feeding force remains weak since the sheet feeding pressure is not so high. When the thick sheet having the high rigidity is fed, the sheet feeding apparatus 101 is therefore incapable of advancing the sheet along the separating slant surface plate 122 and may cause improper sheet feeding. The angle xcex1 is an angle which is formed between a line L connecting a center of the driving shaft 113, that is, a center of the driving gear 112, to a center of the sheet feeding roller 105 and a top surface of the uppermost sheet.
When a small number of sheets are mounted on the sheet feeding tray 121, in contrast, a large angle xcex1 is formed between the sheet feeding arm 114 and the sheet feeding tray 121 as shown in FIG. 11. The sheet feeding roller 105 is subjected to a reaction force F of the sheet feeding force when rotated. This reaction force F is divided into a component of force N in the direction of the sheet feeding pressure and a component of force T in the direction along the longitudinal direction of the sheet feeding arm 114. However, the force N in the sheet feeding pressure is strong since the above described angle xcex1 is large. Accordingly, the sheet feeding pressure is high. The sheet feeding pressure is higher than required and a torque required for driving the sheet feeding roller 105 is also larger than required. As a result, the motor may be out of adjustment, thereby causing improper sheet feeding.
An object of the present invention is to provide a sheet feeding apparatus which is capable of enhancing performance for separating and feeding a thick sheet having high rigidity even when a sheet feeding arm and the sheet form a small angle for feeding the thick sheet having the high rigidity.
Another object of the present invention is to provide a sheet feeding apparatus which is capable of further enhancing performance for separating and feeding a thick sheet having high rigidity even when a sheet feeding arm and the sheet form a large angle for feeding the thick sheet having the high rigidity,
A sheet feeding apparatus according to the present invention is characterized by comprising:
sheet supporting means for supporting a sheet;
a driving rotary member which is driven and rotated by a driving source;
a rocking member which is disposed on the above described driving rotary member so as to be capable of rocking;
a sheet feeding rotary member which is rotatably disposed on the above described rocking member and can be brought into contact with the sheet supported by the above described sheet support means;
a following rotary member which is coupled with the above described sheet feeding rotary member on the above described rocking member;
a winding and hanging member hung around the above described driving rotary member and the above described following rotary member;
a separating slant surface which is disposed on a side of leading ends of sheets fed out by the above described sheet feeding rotary member and functions to separate the sent sheets one by one; and
rocking force generating means which is disposed on the above described rocking member, guides a tensed side of the above described winding and hanging member in a direction to separate this side from the above described sheets, and for generating a rocking force, on said rocking member, which brings the above described sheet feeding rotary member into pressure contact with the sheet by receiving a pressing force produced due to a tension of the tensed side.